The Cistern of Philoxenos (Greek: Κινστέρνα Φιλοξένου), or Binbirdirek Cistern, is a man-made subterranean reservoir in Istanbul, situated between the Forum of Constantine and the Hippodrome of Constantinople in the Sultanahmet district. It has been restored and is now visited as a tourist attraction. The entrance is located at İmran Öktem Sokak 4. 

The cistern was constructed under a palace, often identified as the Palace of Antiochos in the 5th century.  The reservoir has a surface area of 3640 m2, storing 40,000 m3 of water.  The cistern is composed of a large hypostyle chamber supported by vaults.  The 224 columns, each 14 to 15 meters tall, are made of marble from nearby Marmara Island. Each column is a superposition of two columns, joined by a marble ring. The floor of the cistern was later reinforced, so that only the upper column and a short sleeve of the lower column are thus visible. A part of the original exterior is now preserved on display near the entrance. Most of the columns, and also the caps, are engraved with a Greek mason's mark. 

http://en.wikipedia.org/wiki/Cistern_of_Philoxenos